1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus in which pulsed Doppler signals are compensated in accordance with a propagation distance of reflected ultrasonic beams, i.e., a depth of a body under examination.
2. Description of Prior Art
Very recently, a Doppler detection of pulsed ultrasound has been used to map fluid flows in blood vessels within a body, as described in IEEE TRANSACTIONS ON SONICS AND ULTRASONIC, VOL. SU-17, No. 3, JULY 1970 entitled "Pulsed Ultrasonic Doppler Blood-Flow Sensing."
A basic idea of the conventional ultrasonic imaging apparatus utilizing the Doppler effect is first to transmit pulsed ultrasonic beams toward an object, e.g., the body, by an ultrasonic transducer; Secondly, and then to receive scattered ultrasonic echoes which are Doppler-shifted by the blood-flows in the body by the same transducer, thereby converting them into ultrasonic echo signals represents as electronic signals. A continuous wave (CW) signal having the same fundamental frequency as that of a center frequency (e.g., 2.4 MHz) of the ultrasonic echo signals is mixed with the ultrasonic echo signals, thereby performing phase detection by a low pass filter.
The detected phase difference signal is sampled and passed through a band-pass filter to obtain a Doppler shift signal based on movements of blood-flow in the object.
The Doppler shift signal is frequency-analyzed, e.g., by a Fourier transform device, to obtain power distribution of a spectrum, i.e., a spectrum of a given sample volume having a predetermined power of blood flow at a desired position. The power distribution is displayed on a display device in such a manner that time elapse is plotted along the abscissa, a Doppler shift frequency (proportional to a blood flow rate) is plotted along the ordinate, and an intensity of the power distribution of the spectrum is represented by the brightness.
In the ultrasonic imaging apparatus of this type, the amplitudes of the ultrasonic echo signals reflected from portions of the body decrease in correspondence with time elapse, due to attenuation in the depth of the body and dispersion of the ultrasonic echoes. Therefore, these amplitudes gradually become small in accordance with the distances between the portions within the body and the transducer over which the reflected ultrasonic beams (echoes) are propagated. For this reason, in order to obtain useful Doppler shift data, the amplification ratio of the receiver must be changed in conformance with time elapse or with the depth direction of the body in correspondence with a decrease in the amplitude of the reflected echo signal.
However, the center frequency of the ultrasonic echo signal varies widely, e.g., 2 to 5 MHz, and a dynamic range thereof falls in the order of 100 dB. Therefore, the receiver for receiving such ultrasonic echo signals must contend with technical problems, resulting in complex circuitry.
The present invention has been made in consideration of the above situation, and has as its object to provide an ultrasonic imaging apparatus in which an amplitude of an ultrasonic echo signal having wide dynamic range and frequency characteristics can be easily compensated with the propagation distances of the reflected echoes.